1. Field of the Invention
The present invention relates to an optical scanning apparatus that is configured to scan light emitted from a light source on a scanning object, and an imaging apparatus that includes such an optical scanning apparatus. The present invention also relates to a dot position correcting apparatus, an optical scanning apparatus, and an imaging apparatus such as a laser printer or a digital copying machine and a color imaging apparatus that use such dot position correcting apparatus and optical scanning apparatus.
2. Description of the Related Art
An imaging apparatus such as a laser printer or a digital copier may be configured to focus light emitted from a light source that is modulated according to image information on a photoconductor (scanning object) via a polygon mirror and a scanning optical system such as a scanning lens, and scan the light on the surface of the photoconductor in a predetermined direction (main scanning direction) to form a latent image (electrostatic image) on the photoconductor. Then, the imaging apparatus may apply toner to the latent image to develop the image.
FIG. 1 is a diagram showing a typical configuration of an optical scanning apparatus used in an imaging apparatus such as a laser printer or a digital copier that performs an electrophotographic imaging process. The optical scanning apparatus 400 shown in FIG. 1 includes a semiconductor laser unit 401 corresponding to a light source, a polygon mirror 402 corresponding to a rotating deflector, a scanning lens (fθ lens) 403, a photoconductor 404 corresponding to a scanning medium, a photodetector 405, a phase synchronizing circuit 406, a clock generating circuit 407, an image processing unit 408, and a laser drive circuit 409, for example. In this optical scanning apparatus 400, laser emitted from the semiconductor laser 401 is deflected and scanned by the polygon mirror 402 to be irradiated on the photoconductor 404 via the scanning lens 403 to form a beam spot, and in turn, the photoconductor 404 is scanned by the beam spot in the main scanning direction to thereby be exposed. The photoconductor 404 is rotated by a drive unit (not shown) and evenly charged by a charger (not shown) after which the laser exposure process as is described is performed so that a latent image is formed on the photoconductor 404. Then, the latent image is developed by a developer (not shown) to form a toner image. The toner image formed on the photoconductor 404 is transferred onto transfer paper by a transfer unit (not shown), and the transferred toner image is fixed to the transfer paper by a fixing unit (not shown) after which the transfer paper with the toner image fixed thereon is discharged.
The photodetector 405 positioned in front of the image forming region of the photoconductor 405 is configured to detect laser being irradiated from the polygon mirror 402 via the scanning lens (fθ lens) 403. The phase synchronizing circuit 406 is configured to generate an image clock (pixel clock) that is set to a phase in synch with an output signal of the photodetector 405 based on a clock output from the clock generating circuit 407 with respect to each line, and supply the generated image clock to the image processing unit 408 and the laser drive circuit 409. The laser drive circuit 409 is configured to control the formation of the latent image on the photoconductor 404 by driving the semiconductor laser unit 401 according to image data generated by the image processing unit 408 and the image clock that is phase-controlled with respect to each line by the phase synchronizing circuit 6, and controlling the semiconductor laser light emitting time of the semiconductor laser unit 401.
In recent years and continuing, with the growing demand for higher printing speed and higher image quality in the imaging apparatus, techniques have been developed for increasing the rotational speed of the polygon mirror, and increasing the frequency of a clock signal (e.g., pixel clock) used for modulating the light from the light source in order to meet such demands, for example. However, the effects of increasing the printing speed obtained by these techniques are limited, and in turn, a so-called multi-beam scheme involving the use of plural light sources has been developed for responding to the demand for a further increase in the printing speed (e.g., see Japanese Laid-Open Patent Publications No. 2001-272615, No. 2003-72135, and No. 2001-350111).
Also it is noted that variations in the distance from the axis of rotation to the deflection reflective surface of a deflector such as the polygon mirror may cause scanning speed variations in the scanning speed of the beam spot (scanning beam) being scanned on a scanning surface. Such scanning speed variations may cause image drifts that may lead to image quality degradation. Thus, scanning deviations have to be corrected in order to obtain high image quality.
In this respect, a light source that emits plural laser beams (multi-beam laser) so that the plural laser beams are irradiated via the scanning lens (fθ lens) to simultaneously scan a scanning surface with respect to the main scanning direction may be used as the semiconductor laser unit in order to respond to the demand for speed increase and improved image quality in the imaging apparatus, for example. By employing the multi-beam optical scanning scheme as is described above, the rotational speed of the polygon mirror corresponding to the deflector and the pixel clock frequency may be reduced so that high-speed and stable optical scanning and imaging may be realized.
It is noted that a laser diode (LD) array made up of plural single beam laser chips or a single laser chip having plural laser elements as light sources may be used as the light source for generating the multi-beam laser. Further, the light source may be a VCSEL (Vertical Cavity Surface Emitting Laser) that emits laser in a vertical direction, for example.
A multi-beam semiconductor laser such as a LD array is small in size and capable of performing direct modulation at high speed using a drive current. Accordingly, such a multi-beam semiconductor laser is currently being widely used as the light source for a laser printer. However, it is noted that the relationship between the drive current of the semiconductor laser and the light output may vary depending on the temperature. Therefore, there may be difficulties in setting the light intensity of the semiconductor laser to a desired value. It is particularly noted that in the case of using a VCSEL (Vertical Cavity Surface Emitting Laser), temperature change and temperature crosstalk may easily occur in response to light emission and light extinction since the distance between the light sources is relatively small, and thereby, the laser may be vulnerable to light intensity fluctuation.
Also, it is noted that in the case of using a multi-beam optical system in which chromatic aberration of a scanning lens is not corrected, exposure position deviations may occur when there are differences in the oscillating wavelength between light sources, and the scanning width of the light sources for scanning spots on a scanning medium may vary depending on each light source. In this case, the scanning width has to be corrected in order to prevent image quality degradation.
For example, Japanese Laid-Open Patent Publication No. 2001-272615 discloses an optical scanning apparatus including plural light sources in a two-dimensional arrangement that scans the surface of a scanning medium by deflecting plural light fluxes with a deflector, the optical scanning device being configured to maximize the density of light emitting points while avoiding influence from crosstalk that may be caused by heat generated between the light emitting points.
Japanese Laid-Open Patent Publication No. 2003-72135 discloses an imaging apparatus with a surface-emitting laser which imaging apparatus includes means for varying the light intensity of each laser chip in pixel units and means for controlling the light emitting time to thereby control the electrostatic latent images of the pixels.
Japanese Laid-Open Patent Publication No. 2001-350111 discloses an imaging apparatus with a surface-emitting laser which imaging apparatus adjusts the arrangement of light sources to prevent the occurrence of heat stroke and realize high density image recording at the same time.
It is noted that in a typical optical scanning apparatus that uses a light source unit having plural light sources such as a surface-emitting laser, each light source is configured to form one pixel, and thereby, variations in the light emitting levels of the light sources may directly lead to variations in image density. It is particularly noted that measures for correcting variations of pixels with respect to the sub scanning direction do not exist.
Also, in a case where one light source is degraded, the reduction in the light emitting level of the degraded light source directly leads to a decrease in image density.
Also, deviations in the scanning position of a beam spot (scanning beam) scanned on a photoconductor and variations in the scanning speed may occur due to face tangle errors of the polygon mirror and variations in the distance from the rotational axis of the polygon mirror to its deflection reflective surface, for example. Such scanning position deviations and scanning speed variations may cause degradation of image quality.
As is described above, there is an increasing demand for higher image quality in response to the dramatic increase in the resolution of the digital camera, for example; however the apparatuses disclosed in the above references cannot adequately satisfy such a demand for higher image quality without increasing manufacturing costs. It is particularly noted that the demand for higher image quality cannot be adequately satisfied by the above-disclosed apparatuses owing to the fact that pixel position deviations with respect to the sub scanning direction are not properly taken into account.